The present invention relates to an integral rubber article having an electrically insulating part and an electrically conductive part and a method for the preparation of such a rubber article. More particularly, the invention relates to a vulcanized silicone rubber article having an electrically insulating part and an electrically conductive part which serves as an antistatic functional component of an electric or electronic instrument kept prolongedly under compression or under repetition of compression and pressure-release, of which the conductive part serves, for example, as a fixed or movable electrical contacting point as well as an efficient and reliable method for the preparation of such a rubber article having portions of different properties integrated together.
Vulcanized rubber articles having an electrically insulating part and an electrically conductive part integrated together are widely used as a functional component in various kinds of electronic instruments such as a so-called rubber contact as a part of pocketable or desk-top calculators, telephones, remote controllers of television sets, terminals of computers and the like, interconnectors for electrically connecting terminal electrodes on two circuit boards or on a display unit and a circuit board for driving the display unit, antistatic paper-feed rubber rollers in photocopying machines, facsimile machines, word processors and the like, rubber-made platen rollers, rubber-made contact-point rollers, electromagnetic shielding materials and so on.
A method is proposed in Japanese Patent Publication No. 56-41417 for the preparation of an integral rubber article having an electrically insulating part and an electrically conductive part, according to which either one of the insulating and conductive rubber compounds is subjected to vulcanization by use of an organic peroxide having an apparent activation energy of at least 33 kcal/mole as a vulcanizing agent to effect firm bonding to the other. Japanese Patent Publications Nos. 61-39188 and 61-34982 propose that the conductive rubber compound is freed from the trouble of vulcanization inhibition by the carbon black compounded therein as a conductivity-imparting agent when the vulcanizing agent is a non-acyl type organic peroxide or the rubber compound is a silicone rubber curable by the mechanism of the hydrosilation reaction in the presence of a platinum catalyst, referred to as an addition-type silicone rubber hereinbelow, capable of being cured at a relatively low temperature within a relatively short time so as to decrease the energy consumption and increase the productivity.
The above mentioned silicone based integral rubber article having an electrically insulating part and an electrically conductive part is advantageous in respect of the reliableness in the adhesive bonding of the two different rubber parts and the productivity though with several disadvantages. When either one or both of the insulating and conductive parts are made from an addition-type silicone rubber, for example, a serious problem is encountered by using the rubber part prolongedly under heavy compression or with frequent repetition of compression or deformation and release of the compressive or deforming force for several millions of times or more that a large permanent compression set is caused in the rubber part so that the rubber article no longer works as a rubbery elastomeric body with a consequent great decrease in the performance as a functional component. Namely, such a silicone-based integral rubber article in the prior art is unsatisfactory in respect of the behavior of permanent compression set without decreasing the reliability in the adhesive bonding of the two parts. In rubber contacts of the prior art, for example, the contacting point made of the conductive rubber is compressed and depressed repeatedly by being brought into contact with the electrode made of a copper foil or a conductive paste having a certain hardness and thickness to cause a stress at the interface between the parts of the conductive and insulating rubber parts leading to eventual separation of the parts or failure of electrical connection unless the pushing pressure on the push button is substantially increased. At least, it is unavoidable that the finger touch on the key board switch is subject to a change due to the increase in the necessary stroke in the long run of use as a result of the permanent compression set caused in the contacting point.
In a rubber-made interconnector used by being press-held between the terminal electrodes on a display unit and a circuit board for driving the display unit, the contact resistance between the electrodes and the interconnector is reduced with an increase in the contacting pressure by utilizing the elastic resilience of the rubber-made interconnector under compression. When deformation or permanent compression set is caused in the interconnector, the contacting pressure may be decreased consequently with an increase in the contact resistance which is responsible for the decrease in the quality of the display on the display unit. When the display unit is constructed with a substrate made of a material susceptible to fracture, such as a glass plate, supported by utilizing the elastic resilience of the rubber-made interconnector, the permanent compression set caused in the interconnector may eventually lead to a loss of the holding security of the glass substrate which is then subject to a damage by a mechanical impact or shock. Even when the display unit is not destroyed by the mechanical shock, it is sometimes unavoidable that failure in contacting is caused between the terminal electrodes and the interconnector due to relative displacement therebetween as a result of the mechanical shock, especially, when the arrangement pitch of the terminal electrodes on the display unit is so fine as to be 0.7 mm or smaller leading to an error in the display.
As in an antistatic rubber roller which is rotated under a permanently lasting compression in a radial direction, the portion of the rubber under compression is perpetually in movement under the squeezing force as the roller is rotated. Such a condition is very severe for the rubber roller to maintain the rubbery elasticity so that the rubber roller can no longer serve to smoothly and reliably feed sheets of paper to the machine when deformation or permanent compression set has been caused in the rubber roller. It is sometimes unavoidable that a failure is caused in the adhesive bonding between the conductive and insulating rubber-made parts so that the rotation of the core shaft can no longer be transmitted reliably to the outer rubber layer. In a rubber-made platen roller in printers, any depression on the surface of the rubber roller naturally results in uneven printing.